The invention relates generally to electrical power transmission systems and, more particularly, to systems and methods for transmitting DC power in electrical power transmission systems.
There is a growing need to deliver power from onshore or offshore platforms to subsea electrical equipment operating at seabed or remote offshore locations. However, transmission of the electrical power to the sub-sea electrical equipment often requires high voltage power to be transmitted over long distances.
Alternating current (AC) transmission systems are considered inefficient for transmitting high voltage power over long distances as capacitance in AC transmission cable reduces the load carrying capability of the AC cable. In contrast, high voltage direct current (HVDC) transmission provides a more efficient way to transmit high voltage power over long distances. However, strict limitations on amplitude of harmonic currents flowing in the HVDC transmission cable have to be maintained.
In one conventional approach for keeping the amplitude of the harmonic current within required limits in an HVDC transmission, pulse width modulation (PWM) converters are used to control the harmonic current. The inventors of the present invention have found that conventional switching methods of PWM converters in HVDC embodiments lead to excessive switching losses and herein present embodiments wherein such switching losses are reduced.